[Xen-devel] [PATCH V3] vmx/nmi: Do not use self_nmi() in VMEXIT handler

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The self_nmi() code cause's an NMI to be triggered by sending an APIC
message to the local processor.  However, NMIs are blocked by the
VMEXIT, until the next iret or VMENTER.

Volume 3 Chapter 27 Section 1 of the Intel SDM states:

An NMI causes subsequent NMIs to be blocked, but only after the VM exit
completes.

As a result, as soon as the VMENTER happens, an immediate VMEXIT
happens as a result of the queued NMI.  We have seen hundreds of
iterations of this VMEXIT/VMENTER loop before the HVM guest resumes
normal operation.

Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

--
Changes since v2
 * Switch from 'int $2' to do_nmi()
 * Reworked commit message to more clearly explain the problem

diff -r 2489c2926698 -r d7ea938044ac xen/arch/x86/hvm/vmx/vmx.c
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2269,6 +2269,14 @@ void vmx_vmexit_handler(struct cpu_user_
         vector = intr_info & INTR_INFO_VECTOR_MASK;
         if ( vector == TRAP_machine_check )
             do_machine_check(regs);
+        else if ( vector == TRAP_nmi &&
+                ( (intr_info & INTR_INFO_INTR_TYPE_MASK) ==
+                  (X86_EVENTTYPE_NMI << 8) ) )
+            /* Must be called before interrupts are enabled to ensure
+             * the NMI handler code is run before the first IRET. The
+             * IRET unblocks subsequent NMI's (Intel SDM Vol 3, 6.7.1)
+             */
+            do_nmi();
         break;
     case EXIT_REASON_MCE_DURING_VMENTRY:
         do_machine_check(regs);
@@ -2442,7 +2450,6 @@ void vmx_vmexit_handler(struct cpu_user_
                  (X86_EVENTTYPE_NMI << 8) )
                 goto exit_and_crash;
             HVMTRACE_0D(NMI);
-            self_nmi(); /* Real NMI, vector 2: normal processing. */
             break;
         case TRAP_machine_check:
             HVMTRACE_0D(MCE);

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